This invention relates to the need for alleviating the growing environmental problem of excessive plastic waste that makes up an increasing volume fraction of materials in landfills. Biodegradable polymers and products formed from biodegradable polymers are becoming increasingly important in view of the desire to reduce the volume of solid waste materials generated by consumers each year. The invention further relates to the need for developing new plastic materials that can be used in applications where biodegradability, compostability or biocompatibility are among primary desirable features of such applications, particularly under anaerobic conditions. There have been many attempts to make degradable articles. However, because of costs, the difficulty in processing, and end-use properties, there has been little commercial success. Many compositions that have excellent degradability have only limited processability. Conversely, compositions which are more easily processable have reduced degradability.
A conventional disposable absorbent product is already to a large extent compostable. A typical disposable diaper, for example, consists of about 80% of compostable materials, e.g., wood pulp fibers, and the like. Nevertheless, there is a particular need to replace polyethylene backsheets in absorbent articles with liquid impervious films of compostable material, because the backsheet is typically one of the largest non-compostable components of a conventional disposable absorbent article.
To produce films that have more acceptable end-use properties, choosing acceptable degradable polymers is challenging. The degradable polymers should be thermoplastic such that conventional film processing methods can be employed, including running on converting lines. Further, it is important that the film or large film fragments undergo an initial breakup to much smaller particles during the initial stages of composting.
In addition, there has been an emerging interest in the breathability of disposable hygiene products to minimize the discomfort associated with the accumulation of high humidity. Breathable films that can contain liquid while allowing some passage of moisture vapor are of special interest in constructing such products. Controlling the pore size is achieved by dispersing filler particles uniformly and very finely within the film matrix before a stretching operation. Materials such as polyolefin have such a low affinity to filler surface that it is difficult to obtain a good dispersion of particles. Polyesters have a better affinity to many solid surfaces so that particles tend to spread more easily, however, if the interaction is too strong, the desired mechanical failure at the interface between the filler and film matrix to create pores during the stretching will not occur. Materials with a moderate level of interaction with fillers are needed for breathable films. Further, such materials must be substantially ductile to prevent macroscopic mechanical failure leading to large tears during the stretching. For example, typical aromatic polyesters such as polyethylene terephthalate are too brittle to contain the localized mechanical failure around the individual filler particles.
Polyhydroxyalkanoates (PHAs) are generally semicrystalline, thermoplastic polyester compounds such as isotactic poly(3-hydroxybutyrate) or PHB, and isotactic poly(3-hydroxybutyrate-co-valerate) or PHBV. Both copolymers suffer the drawbacks of high crystallinity and fragility/brittleness. Due to the slow crystallization rate, a film made from PHBV will stick to itself even after cooling; a substantial fraction of the PHBV remains amorphous and tacky for long periods of time. In both cast film operations and in blown films, residual tack limits processing. Medium to long side-chain PHAs, such as isotactic polyhydroxyoctanoates (PHOs), are virtually amorphous owing to the recurring pentyl and higher alkyl side-chains. When present, their crystalline fraction however has a very low melting point as well as an extremely slow crystallization rate. For example, Gagnon, et al. in Macromolecules, 25, 3723–3728 (1992), incorporated herein by reference, show that the melting temperature is around 61° C. and that it takes about 3 weeks to reach the maximum extent of crystallization at its optimal crystallization temperature.
Further poly(3-hydroxyalkanoate) copolymer compositions have been disclosed by Kaneka (U.S. Pat. No. 5,292,860) and Procter & Gamble (U.S. Pat. Nos. 5,498,692; 5,536,564; U.S. Pat. No. RE 36,548; U.S. Pat. Nos. 5,685,756; 5,942,597; 5,990,271; 6,160,199). All describe various approaches of tailoring the crystallinity and melting point of PHAs to any desirable lower value than in the high-crystallinity PHB or PHBV by randomly incorporating controlled amounts of “defects” along the backbone that partially impede the crystallization process. The semicrystalline copolymer structures can be tailored to melt in the typical use range between 80° C. and 150° C. and are less susceptible to thermal degradation during processing. In addition, the biodegradation rate of these copolymers is higher as a result of their lower crystallinity and the greater susceptibility to microorganisms. Yet, whereas the mechanical properties and melt handling conditions of such copolymers are generally improved over that of PHB or PHBV, their rate of crystallization is characteristically slow, often slower than PHB and PHBV.
In general, it has been a considerable challenge to convert these newer PHA copolymers, as well as other biodegradable polymers, into useful forms by conventional melt methods. The polymers remain substantially tacky after they are cooled down from the melt and remain as such until sufficient crystallinity sets in, particularly with PHA copolymers with noncrystallizing component levels above 10 wt %. Residual tack typically can lead to material sticking to itself or to the processing equipment, or both, and thereby can restrict the speed at which a polymeric product is produced or prevent the product from being collected in a form of suitable quality.
To produce environmentally degradable articles, attempts have been made to process natural starch on standard equipment using existing technology known in the plastics industry. Since natural starch generally has a granular structure, it needs to be “destructured” before it can be melt processed. Modified starch (alone or as the major component of a blend) has been found to have poor melt extensibility, resulting in difficulty in successful production of fibers, films, foams or the like.
To produce films or laminates that have more acceptable processability and end-use properties, biodegradable polymers need to be combined with starch. Selection of a suitable biodegradable polymer that is acceptable for blending with starch is challenging. The biodegradable polymer must have a suitable melting temperature. The melting temperature must be high enough for end-use stability to prevent melting or structural deformation, but not too high of a melting temperature to be able to be processable with starch without burning the starch. These requirements make selection of a biodegradable polymer to produce starch-containing films very difficult. Further, the blend must be processable on conventional film making equipment.
U.S. Pat. No. 5,874,486 is to polymeric compositions comprising a matrix including a starch component and at least one of certain synthetic thermoplastics polymeric components in which a filler is dispersed and including a fluidising agent. U.S. Pat. No. 6,117,925 is to mixtures of transesterification products of starch and certain hydrophobic polymers. U.S. Pat. No. 6,096,809 is to compositions including thermoplastic starch and at least one of certain polymers wherein the water content of the composition is less than 1% while in a melted state. U.S. Pat. No. 5,844,023 is to a polymer dispersion consisting essentially of a mixture of thermoplastic starch and at least one of certain polymers and a phase mediator molecularly coupling the two phases. Improvements can be made in the miscibility, processibility, texture, feel, and tackiness of these prior art compositions.
For breathable film fabrication, there is a need to develop environmentally degradable materials that have a moderate affinity for solid filler surfaces for good particle dispersion and that are soft and ductile to have only localized mechanical failure to create fine pores upon stretching.